The present invention relates to apparatus for substantially reducing warpage of relatively large area, relatively thin webs of substrate material onto which amorphous silicon alloy layers are deposited by continuous glow discharge production techniques performed within dedicated deposition chambers. More particularly, in the continuous production glow discharge deposition apparatus of the prior art, the web of substrate material is moved, under tension, from a supply core through a plurality of deposition chambers wherein successive amorphous alloy layers are deposited. While moving through the plurality of deposition chambers and onto a take-up core, the web of substrate material assumes a normal, elongated path of travel. Due, inter alia, to (1) the elevated temperature required for the deposition of amorphous semiconductor alloy layers onto the web, (2) the gravitational forces acting on the web, (3) the normal elongated path of travel through the deposition chambers which the web follows, and (4) stress on the web, itself, the web of substrate material experiences longitudinal and transverse warpage. The apparatus of the present invention is adapted to establish a plurality of substantially equally spaced magnetic fields within each of the plurality of deposition chambers for urging the web of substrate material out of its normal, sagging elongated path of travel through the plurality of deposition chambers and into a substantially planar path of travel. In this manner, the web of substrate material is "flattened" so as to provide a substantially planar deposition surface onto which uniform amorphous alloy layers may be deposited. The result is the production of improved, more efficient large area photovoltaic devices.
Recently, considerable efforts have been made to develop processes for depositing amorphous semiconductor alloys, each of which can encompass relatively large areas, and which can be doped to form p-type and n-type materials for the production of p-i-n-type devices, which are, in operation, substantially equivalent to their crystalline semiconductor counterparts. For many years such work with amorphous silicon or germanium films was substantially unproductive because of the presence therein of microvoids and dangling bonds which produce a high density of localized states in the energy gap. Initially, the reduction of the localized states was accomplished by glow discharge deposition of amorphous silicon films wherein silane (SiH.sub.4) gas is passed through a reaction tube where the gas is decomposed by a radio frequency (r.f.) glow discharge and deposited on a substrate at a substrate temperature of about 500-600 degrees K. (227-327 degrees C.). The material so deposited on the substrate is an intrinsic amorphous material consisting of silicon and hydrogen. To produce a doped amorphous material, phosphine gas (PH.sub.3), for n-type conduction, or diborane (B.sub.2 H.sub.6) gas, of p-type conduction is premixed with the silane gas and passed through the glow discharge reaction tube under the same operating conditions. The material so deposited includes supposedly substitutional phosphorus or boron dopants and is shown to be extrinsic and of n or p conduction type. The hydrogen in the silane was found to combine, at an optimum temperature, with many of the dangling bonds of the silicon during the glow discharge deposition to reduce the density of the localized states in the energy gap, thereby causing the amorphous material to more nearly approximate the corresponding crystalline material.
It is now possible to prepare greatly improved amorphous silicon alloys, that have significantly reduced concentrations of localized states in the energy gaps thereof, while providing high quality electronic properties by glow discharge. This technique is fully described in U.S. Pat. No. 4,226,898, Amorphous Semiconductors Equivalent to Crystalline Semiconductors, Stanford R. Ovshinsky and Arun Madan which issued Oct. 7, 1980 and by vapor deposition as fully described in U.S. Pat. No. 4,217,374, Stanford R. Ovshinsky and Masatsugu Izu, which issued on Aug. 12, 1980, under the same title. As disclosed in these patents, fluorine introduced into the amorphous silicon semiconductor operates to substantially reduce the density of the localized states therein and facilitates the addition of other alloying materials, such as germanium.
Activated fluorine readily diffuses into, and bonds to, amorphous silicon in a matrix body to substantially decrease the density of localized defect states therein. This is because the small size of the fluorine atoms enables them to be readily introduced into an amorphous silicon matrix. The fluorine bonds to the dangling bonds of the silicon and forms a partially ionic stable bond with flexible bonding angles, which results in more stable and more efficient compensation or alteration than could be formed by hydrogen, or other compensating or altering agents which were previously employed. Fluorine is considered to be a more efficient compensating or altering element than hydrogen when employed alone or with hydrogen, because of its exceedingly small size, high reactivity, specificity in chemical bonding, and high electronegativity.
Compensation may be achieved with fluorine, alone or in combination with hydrogen, upon the addition of such element(s) in very small quantities (e.g., fractions of one atomic percent). However, the amounts of fluorine and hydrogen must desirably used are much greater than such small percentages, permitting the elements to form a silicon-hydrogen-fluorine alloy. Thus, alloying amounts of fluorine and hydrogen may, for example, be used in a range of 0.1 to 5 percent or greater. The alloy thus formed has a lower density of defect states in the energy gap than can be achieved by the mere neutralization of dangling bonds and similar defects states. In particular, it appears that use of larger amounts of fluorine participates substantially in effecting a new structural configuration of an amorphous silicon-containing material and facilitates the addition of other alloying materials, such as germanium. Fluorine, in addition to the aforementioned characteristics, is an organizer of local structure in the silicon-containing alloy through inductive and ionic effects. Fluorine, also influences the bonding of hydrogen by acting to decrease the density of the defect states which hydrogen normally contributes. The ionic role that fluorine plays in such an alloy is an important factor in terms of the nearest neighbor relationships.
The concept of utilizing multiple cells, to enhance photovoltaic device efficiency, was discussed at least as early as 1955 by E. D. Jackson, U.S. Pat. No. 2,949,498 issued Aug. 16, 1960. The multiple cell structures therein discussed utilized p-n junction crystalline semiconductor devices. Essentially the concept is directed to utilizing different band gap devices to more efficiently collect various portions of the solar spectrum and to increase open circuit voltage (Voc). The tandem cell device has two or more cells with the light directed serially through each cell, with a large band gap material followed by a smaller band gap material to absorb the light passed through the first cell or layer. By substantially matching the generated currents from each cell, the overall open circuit voltage is increased without substantially decreasing the short circuit current.
Many publications on crystalline stacked cells following Jackson have been reported and, more recently, several articles dealing with Si-H materials in stacked cells have been published. Marfaing proposed utilizing silane deposited amorphous Si-Ge alloys in stacked cells, but did not report the feasibility of doing so. (Y. Marfaing, Proc. 2nd European) Communities Photovoltaic Solar Energy Conf., Berlin, West Germany, p. 287, (1979).
Hamakawa et al., reported the feasibility of utilizing Si-H in a configuration which will be defined herein as a cascade type multiple cell. The cascade cell is hereinafter referred to as a multiple cell without a separation or insulating layer therebetween. Each of the cells was made of an Si-H material of the same band gap in a p-i-n junction configuration. Matching of the short circuit current (J.sub.sc) was attempted by increasing the thickness of the cells in the serial light path. As expected, the overall open circuit voltage (Voc) of the device increased and was proportional to the number of cells utilized.
Due to the beneficial properties attained by the introduction of fluorine, amorphous alloys used to produce cascade type multiple cells now incorporate fluorine to reduce the density of localized states without impairing the electronic properties of the material. Further band gap adjusting element(s), such as germanium and carbon, can be activated and are added in vapor deposition, sputtering or glow discharge processes. The band gap is adjusted as required for specific device applications by introducing the necessary amounts of one or more of the adjusting elements into the deposited alloy cells in at least the photocurrent generation region thereof. Since the band gap adjusting element(s) has been tailored into the cells without adding substantial deleterious states, because of the influence of fluorine, the cell alloy maintains high electronic qualities and photoconductivity when the adjusting element(s) are added to tailor the device wavelength characteristics for a specific photoresponse application. The addition of hydrogen, either with fluorine or after deposition, can further enhance the fluorine compensated or altered alloy. The post deposition incorporation of hydrogen is advantageous when it is desired to utilize the higher deposition substrate temperatures allowed by fluorine.
It is of obvious commercial importance to be able to mass produce photovoltaic devices. Unlike crystalline silicon which is limited to batch processing for the manufacture of solar cells, amorphous silicon alloys can be deposited in multiple layers over relatively large area substrates to form solar cells in a high volume, continuous processing system. Continuous processing systems of this kind are disclosed, for example, in pending patent applications: Ser. No. 151,301, filed May 19, 1980 for A Method of Making P-Doped Silicon Films and Devices Made Therefrom; Ser. No. 244,386, filed Mar. 16, 1981 for Continuous Systems For Depositing Amorphous Semiconductor Material; Ser. No. 240,493, filed Mar. 16, 1981 for Continuous Amorphous Solar Cell Production System; Ser. No. 306,146, filed Sept. 28, 1981 for Multiple Chamber Deposition and Isolation System and Method; and Ser. No. 359,825, filed Mar. 19, 1982 for Method and Apparatus For Continuously Producing Tandem Photovoltaic Cells.
As disclosed in these applications, a web of substrate material may be continuously advanced through a succession of deposition chambers, wherein each chamber is dedicated to the glow discharge deposition of a specific amorphous semiconductor alloy material. In making a solar cell of p-i-n-type configuration, the first chamber is dedicated for depositing a p-type amorphous silicon alloy layer onto the substrate, the second chamber is dedicated for depositing an intrinsic amorphous silicon alloy layer atop the p-type layer, and the third chamber is dedicated for depositing an n-type amorphous silicon alloy layer atop the intrinsic layer. In order to deposit amorphous semiconductor alloy layers which will produce efficient photovoltaic devices, it is necessary that each one of the alloy layers be of substantially uniform thickness. It has been determined that the deposition apparatus of the prior art is unable to prevent (1) transverse warping (warping may synonymously be termed buckling or canoeing) of the web of substrate material caused by the force of gravity acting on the substrate, the elongated path of travel which the substrate follows, stresses on the web, itself, the high deposition temperatures to which the substrate is continuously subjected; or (2) longitudinal warping of the web of substrate material which occurs when the tension on the substrate initiated by drive and tensioning motors is not very carefully adjusted. When the substrate warps, peaks and valleys are formed. It is common to have greater thicknesses of amorphous alloy materials deposited in the valleys while none or lesser thicknesses of amorphous alloy materials are deposited at the peaks. Such non-uniform deposition of the amorphous semiconductor alloy layers is undesirable, can create short circuits and generally serves to impair efficiency and operation of the large area photovoltaic devices produced therefrom.
One method of increasing the efficiency of photovoltaic cells produced on relatively large area, relatively thin, elongated webs of substrate material is to eliminate or substantially reduce transverse and longitudinal warpage of webs of substrate material traveling through the deposition chambers. It is to this end that the apparatus of the present invention is directed. More particularly, the present invention contemplates the establishment in each deposition chamber of at least one magnetic field for flattening the web by urging the web into a substantially planar configuration. This is accomplished by moving the magnetic web of substrate material out of its normal elongated path of travel through each of the deposition chambers. The flattening of the web created by the magnetic fields permits substantially uniform amorphous semiconductor alloy layers to be deposited onto the web of substrate material and hence increases the overall efficiency of the large area photovoltaic devices.
These and other objects and advantages of the present invention will become clear from the drawings, the detailed description of the invention and the claims which follow.